Quantity-intensity (Q/I) relationships of NH₄⁺ in purple soils and purple paddy soils different in land use (paddy field, dry land, woodland and vegetable plot) were studied. It was found that in both 0~20 cm and 20~40 cm layers of the two soils the curve of NH₄⁺ Q/I relationships were observed only when NH₄⁺ activity ratios in the soils were low, suggesting release of nonexchangeable (or specifically adsorbed) NH₄⁺ in the soils NH₄⁺ in the two soils ranged from 71.47 to 203.7 cmol kg^-1 (mol L^-1)^-1/2 in potential buffering capacity (PBC), from 0.029 5 to 0.089 7 cmol_c kg^-1 in labile NH4 (-ΔNH₄⁰), from 0.187 × 10^-3 to 1.255 × 10^-3 (mol L^-1)^1/2 in equilibrium activity ratio (AR⁰_NH4) and from 0.010 6~0.118 5 cmol kg^-1 in specific adsorption sites (NH_4-sas). Correlation analysis and path analysis indicate that the content of clay (<0.002 mm) is in extremely significant positive relationship with both PBC and NH_4-sas (p<0.01). The effect of clay (<0.002 mm) is mainly indirect on PBC (indirect path coefficient, 0.585), but strong and direct on NH_4-sas. Positive linear relationships were observed of organic C with -ΔNH₄⁰ and AR⁰_NH4 in all soil samples. The effect of organic C is high and direct on -ΔNH₄⁰ (direct path coefficient, 0.966), but indirect on -ΔNH₄⁰. The concentration of exchangeable NH₄⁺ is positively related to -ΔNH₄⁰ (r=0.876 4, n=8, p<0.01) and they are approximate in value. Moreover, exchangeable NH₄⁺ is also positively related to AR⁰_NH4 (r=0.983 7, n=8, p<0.01). This study clearly demonstrates that differences between the soils in clay and organic C are the main reason for the differences in exchangeable NH₄⁺ and Q/I parameters between purple soils different in land use. The use of exchangeable NH₄⁺ as indicator of NH₄⁺ availability has a similar effect as the use of Q/I relationships.